Thread: cant fix my p420 code

p11 po420 code. new cat, new o2 sensors, plugs and wires. i drove the car 50 miles and the light came back on. what else should i look for? the car is stock. i looked up tsb's and didnt see any for my problem. the light comes on when at cruising speed. low rpms, 2250-2500. any ideas?
its an 2002 sport a/t with 150k on the clock

put a scanner on it that has datastream capabilities, and pay attention to the long term and short term fuel trim, that will let you know more or less whats going on. If its adding fuel then you might have a vacuum leak and the pcm is trying to compensate by adding fuel, overfueling is the term. If its taking away fuel it could be a leaky injector or clogged filter. The fuel trims will def give you an idea though. I use it all the time to quickly diagnose vehicles. Good luck with the troubleshooting, its def one of the hardest things to do in automotive.

should the engine be warmed up or cold?

Do what xavil1320 sugested . I would start with checking the fuel trims with motor completly cold to see if there is a vac leak that could be going away when the car is hot or warmed up like a intake manifold gasket . The fuel trims should be switching from +/- in closed loop. Good way to check vac leaks is use 2+2 carb spray with the straw that comes with the can and check around the intake manifold ect ect watching the fuel trims or sensor voltage . If you see the trims jump -5,10,15 ect then you have a leak or the voltage goes upwards .7-8 then you have a leak . Also you rear o2 should be switching slow vrs the front upstream sensor. Any other codes present or pending like evap or egr ?

this is with the torque app from my phone. engine temps 109.4 front o2 0.1V rear 0.3V ltft1 2.3
stft -2.3